Coquasi-bialgebroids and cocycle twisting

Abstract

We introduce coquasi-bialgebroids over a noncommutative base algebra. Using Takeuchi's \(\times_B\)-coalgebra formalism, we require the coproduct to remain an algebra map into the Takeuchi product, while the product is associative only up to an invertible normalized \(3\)-cocycle. This gives a bialgebroid analogue of coquasi-bialgebras and provides a natural framework for cocycle-twisted bialgebroid constructions. We develop the basic theory and prove a twisting theorem by convolution-invertible \(2\)-cochains. As a main class of examples, we construct coquasi Connes--Moscovici-type bialgebroids on \(B\otimes H\otimes B\), where \(H\) is a coquasi-bialgebra measuring an algebra \(B\), with twisting data \(γ:H\otimes H\to B\). We also give finite-group examples arising from a subgroup \(G\subseteq X\) and a choice of transversal. Finally, under finite projectivity assumptions, we describe the dual quasi-bialgebroid construction and its relation to Drinfeld-type twisting.